| Summary: | The radiative effect due to atmospheric aerosol particles still has large uncertainties. These uncertainties confound interpretation of climate change due to CO2 increase between pre-industrial and present times, and limit our ability to project future climate change. One factor causing these uncertainties is poor representation of the chemical and physical processes related to secondary organic aerosol (SOA) in current models. Therefore, I develop different mechanisms to simulate the SOA formation in a global 3-d model (IMPACT). The basic mechanism includes SOA formation from organic nitrates and peroxides produced from an explicit chemical formulation, using partition coefficients based on thermodynamic principles together with assumptions for the rate of formation of low-volatility oligomers. I also include the formation of low-volatility SOA from the reaction of glyoxal and methylglyoxal on aqueous aerosols and cloud droplets as well as from the reaction of epoxides on aqueous aerosols, using a simple reactive uptake parameterization. In addition, I develop a multiphase process scheme with detail reactions in cloud water and aerosol water to simulate the SOA formation in the aqueous phase. The model using these mechanisms is shown to be able to predict the observed SOA reasonably well. Finally, I use this fully explicit SOA formation model to investigate the change in SOA between present day and pre-industrial conditions and to assess the radiative forcing associated with both anthropogenic and biogenic SOA. The increase of biogenic and anthropogenic SOA results in a global average direct forcing ranging from -0.06 to -0.21 Wm-2 and a first indirect forcing ranging from -0.24 to -0.32 Wm-2, depending on the size distribution and refractive index of SOA. Moreover, the radiative forcing of present-day organic aerosol in snow and sea-ice is estimated and is shown to cause a warming effect comparable to that due to black carbon in snow and sea-ice. PhD Atmospheric, Oceanic and Space Sciences University of ...
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